1. Field of the Invention
The present invention relates to direction finding systems and particularly to such systems of the type employing a pair of spaced antennas by which carrier and sideband waves may be radiated to a remote point and may be received at the remote point and utilized to provide an indication of the direction of the remote point from the spaced antennas. More specifically, this invention relates to the simultaneous one-way transmission of bearing angle information from a single transmitter station to an unlimited number of mobile receivers. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
2. Description of the Prior Art
One-way transmission of pointing or bearing angle information from a transmitter fixed in a local framework of reference to a mobile receiver in the same reference system is well known in the art. Prior art methods for one-way transmission of bearing angle information may be generally classified as scanning fan beam techniques or interferometric techniques.
In the context of an air traffic control system, prior art scanning fan beam techniques employ a transmitted beam of electromagnetic radiation which scans across the coverage sector in both azimuth and elevation. The fan beam in each coordinate is generated by a single antenna to define a spatial/time angular resolution. Scanning beam techniques, whether electronic or mechanical means are employed to achieve the scanning, limit the available data rate since each passage of the beam across the airborne receiver represents a single data point with the time interval to the subsequent data point determined by sector angle coverage requirements and scan rate. Increasing the scan rate to reduce the time reference interval between sequential data points reduces the observing time available for each data point; i.e., the dwell time of the fan beam on the receiver is decreased as the scan rate is increased; and thus has a deleterious effect on the accuracy of the measurement as well as on the bandwidth (amount of spectrum) required by the system.
Interferometric techniques include those systems wherein bearing angle in either coordinate is determined by measuring the path length difference between either end of a fixed baseline to a point, as shown in FIG. 1. In a typical embodiment the point will be a mobile receiver, and a pair of transmitting antennas will be located at either end of the baseline. The bearing angle, .theta., to the mobile receiver measured in the plane containing the baseline and the receiver is given by: EQU .theta. = sin .sup..sup.-1 (L/d) (1)
where:
L = the path length difference between either end of the baseline and the mobile receiver, and PA0 d = the baseline length.
Although the object of most inteferometric techniques is to perform an angle measurement, this is accomplished through the measurement of the path length difference, L, by radiating signals from antennas located at either end and/or along the baseline. By appropriate processing of these signals the receiver is capable of deriving the path length difference, L. Since the value of the baseline length, d, is known, the bearing angle .theta. is uniquely determined by the path length difference measurement in accordance with Equation (1). It is for this reason that interferometric techniques are frequently referred to as self-coded; i.e., an angle related tone is not transmitted as in the case of mechanical scanning beam systems. In interferometric systems each angle relative to a particular baseline bears a unique relationship to the path length difference, measured from each end of the baseline, in that angular direction. The measured path length difference is represented by the surface of a cone with the baseline as the conical axis.
If a second baseline with associated transmitters were placed orthogonal to the first the measurable path length difference would also generate a series of conical surfaces, each corresponding to a specific angle, in which the second baseline is the conical axis of the second set of conical surfaces. The intersection of the conical surfaces represented by two orthogonal baseline transmitter systems uniquely determines the bearing angles of the receiver relative to the orthogonal baselines in the local framework of reference.
The foregoing interferometric principles apply to both conventional prior art techniques as well as to the present invention.
In the conventional prior art interferometric techniques, a carrier signal at a frequency f.sub.c is radiated from an antenna element at one end of a baseline and a sideband signal at f.sub.c + f.sub.m is radiated from an antenna at the other end of the baseline. The modulation frequency, f.sub.m, is also simultaneously radiated on a subcarrier from an antenna located on or near the baseline. At the receiver, the carrier and sideband signals are heterodyned to extract the modulation frequency, f.sub.m. The phase of the modulation frequency resulting from this heterodyning action is shifted relative to the phase of the modulation frequency simultaneously received on the subcarrier by an angular amount determined by the path length difference between either end of the baseline and the receiver. The phase shift occurs as a consequence of the fact that the carrier and sideband signals travel over paths to the receiver that differ in length by an amount corresponding to the observed phase shift. Thus, in the prior art, the measurement of the difference between the phase of the modulation frequency, derived by heterodyning the received carrier and sideband signals, and the phase of the modulation frequency, simultaneously transmitted on a subcarrier, is related to the path length difference L in accordance with the following: EQU L = (.lambda. .sub.c .phi. )/(2.pi.) (2)
where .lambda. .sub.c = the wavelength measured at the carrier frequency. The receiver determines the position angle .theta. through the measurement of the phase difference .phi. from the relationship: ##EQU1## which is obtained directly from Equations (1) and (2).
The important feature to be noted in regard to Equations (2) and (3), and hence to prior art interferometric techniques, is that the phase difference measured by the receiver refers to a corresponding path length difference measured in fractional wavelengths at the carrier frequency even though the phase difference measurement is performed at the modulation frequency. This characteristic of prior art interferometric methods leads to two major restraints and concomitant disadvantages associated with prior art interferometric technique, namely angular ambiguity and low data rate.
Angular ambiguity arises as a consequence of the conflicting requirements associated with minimum baseline length d required to achieve a desired angular resolution, and the maximum separation of transmitting elements allowed to assure a unique bearing angle relationship for each measured value of phase difference .phi.. The latter restraint is fully deduced from the foregoing analysis by noting the presence of a cyclic phase ambiguity associated with path length differences which are integer values of the carrier wavelength; i.e., the measured phase difference .phi. will have the same value for all angles which differ in path length by one wavelength at the carrier frequency. Although means have been developed for resolving such particular ambiguities, such means involve time division multiplexing of the transmitted signal among a number of transmitting antenna elements located along the baseline in addition to the antennas located at either end of the baseline. As a result, the time required by the receiver to resolve the ambiguity and determine the specific angle .theta. is determined by the time needed to process the several time multiplexed signals on their sequential arrival at the receiver. Consequently, as the required sector angle coverage is increased the time required to resolve the angle ambiguity is also increased thus resulting in a reduction in data rate. Prior art means for resolving angle ambiguity are comparatively complex and thus have an additional deleterious effect on system cost and reliability.
An additional restraint imposed on prior art interferometric methods results from errors associated with motion of the receiver relative to the transmitter during the time of position angle measurement. Since the fundamental parameter involved in the measurement of the bearing angle is the phase difference between two frequencies; i.e., a carrier frequency transmitted from one end of the baseline and a carrier plus modulation frequency transmitted from the other end; a Doppler error related to receiver velocity in the direction of the transmitter will be introduced directly on the difference or modulation frequency thereby resulting in a velocity related angle measurement error. To minimize the effect of Doppler errors, it is necessary in prior art interferometric methods to utilize a relatively low modulation frequency, usually in the audio range or at least limited to approximately 100 KHz, since the magnitude of the Doppler error, for a given velocity, is directly proportional to the modulation frequency.
Interferometric techniques do, however, overcome several of the above-discussed disadvantages of scanning fan beam techniques as a consequence of the inherent ability of an interferometric system to simultaneously transmit a signal format over a broad angle coverage sector from which an unlimited number of airborne receivers may determine their position angle relative to the interferometer baseline. Although the angular width of the sector which can be covered by prior art interferometric techniques without encountering angular ambiguity is significantly larger than the width of a scanning fan beam capable of providing equivalent performance, typical air traffic control sector coverage requirements exceed the ambiguity limits of simple prior art interferometric techniques. As discussed above, prior art interferometric methods for overcoming angle ambiguity restraints, in order to provide improved angle sector coverage, have the disadvantage that the antenna systems employed are complex. Additionally, prior art interferometric techniques generally require that critical proportioning of circuit constants be established and maintained between the elements of the transmitting apparatus if satisfactory operation of the direction finding system is to be realized. There is the further disadvantage that the receiving apparatus used in such prior art interferometric systems is relatively complex and therefore expensive.